1. Technical Field
This invention relates to constant velocity stroking joints.
2. Related Art
Constant velocity stroking joints (xe2x80x9cC/V jointsxe2x80x9d) have particular application in front wheel drive systems for automotive vehicles. Power is transmitted from the transmission to the drive wheels via shaft assemblies. The shaft assemblies include C/V joints at their inboard and outboard ends. The inboard C/V joint is coupled to the transmission and the outboard C/V joint is coupled to the drive wheels. During normal operation of a front wheel drive vehicle, the wheels move up and down, and thus provision must be made for both angular and axial movement of the shaft assembly. So-called xe2x80x9cjouncexe2x80x9d and xe2x80x9creboundxe2x80x9d imparted by the vehicle suspension system further necessitates the need for axial stroking or plunging capability of the shaft assemblies.
In a typical front wheel drive system, the outboard C/V joints are designed to accommodate large joint angularity but no axial stroking, whereas the inboard C/V joints exhibit less joint angularity but can stroke axially.
A typical inboard stroking C/V joint for front wheel drive applications includes an outer housing formed with a plurality of circumferentially spaced axial guide channels on its inner surface that align with corresponding axial guide channels of an inner race to define tracks for torque-transmitting balls. The race is adapted to mount on one end of a rigid shaft that is coupled at its opposite end to the outboard C/V joint. An outer cage disposed between the housing and inner race is formed with a plurality of ball pockets in which the balls are received. The outer diameter surface of the cage is in part spherical and engages the inner wall of the housing. The inner diameter surface of the cage is spherical and mates with a corresponding spherical outer diameter surface of the inner race. The inner and outer spherical surfaces have centers of curvature which are offset axially from a center plane of the balls on opposite sides thereof (so-called xe2x80x9cdouble offsetxe2x80x9d cage). Such enables angular movement of the joint while maintaining a constant velocity characteristic.
In such a joint construction, axial stroking is achieved by conjoint axial movement of the inner race and outer cage relative to the outer housing. However, the balls are restrained against rolling movement by the cage, and thus axial stroking requires the balls to skid along the guide channels, which is a cause for unwanted friction, potential noise, and high loading.
It has been proposed to provide limited rolling of the balls in an effort to alleviate the above concerns, but at the expense of disrupting the true constant velocity characteristic of the joints. Full rolling of the balls through the full length of the axial stroke under normal operating conditions is also not achieved by the known proposals, thus offering only a partial solution to the problem. U.S. Pat. Nos. 4,511,346 and 4,573,947 for example disclose double offset joints formed with a large radius spherical profile on the inner surface of the cage that mates with a relatively smaller radius spherical outer surface of the inner race. The mismatch in size of the mating spherical surfaces enables the inner race to move axially relative to the outer cage by a small amount disclosed to be in the range of about 0.5 to 1.5 mm. It is further disclosed that the axial movement of the inner race must be carefully controlled in order to avoid disrupting the torsional characteristics of the joint. It will be appreciated that shifting the inner race axially has the effect of shifting the working center of curvature or pivot location of the mating spherical surfaces of the inner race and outer cage axially toward or away from the ball center plane, which in turn disrupts the true constant velocity characteristic of the joint.
U.S. Pat. No. 5,106,343 takes a similar approach of permitting a small amount of axial shifting to occur between the inner race and outer cage by altering the geometry of the mating surfaces such that they do not truly compliment one another. A small cylindrical surface region is provided along the otherwise spherical inner diameter surface of the outer cage. The outer diameter surface of the inner race remains spherical and now mismatched with respect to the mating surface of the outer cage, permitting the inner race to shift axially relative to the outer cage by a very small amount. This construction shares the same disadvantages as those discussed above with respect to the ""346 and ""947 patents.
The present invention overcomes or greatly minimizes the foregoing disadvantages of prior art C/V joints.
A constant velocity stroking joint comprises a housing having a generally cylindrical inner surface formed with a plurality of circumferentially spaced guide channels therein extending axially of the housing. An inner race has an outer surface formed with a plurality of circumferentially spaced, axially extending guide channels aligned with the guide channels of the housing to define a plurality of ball tracks. A plurality of torque-transmitting balls are disposed within the ball tracks. An outer cage is disposed between the inner race and the housing and is formed with a plurality of ball pockets in which the balls are received. The outer cage has an outer diameter surface that is at least in part spherical engaging the inner surface of the housing to enable the outer cage to move angularly relative to the housing.
According to a characterizing feature of the invention, an intermediate cage is provided between the outer cage and the inner race. The intermediate cage and the outer cage have mating outer and inner diameter spherical surfaces, respectively, which cooperate to permit the intermediate cage and the inner race to move as a unit angularly relative to the outer cage. An inner diameter surface of the inner race mates with an outer diameter surface of the intermediate cage configured to permit the inner race to stroke axially relative to the intermediate cage.
One advantage of the present invention is that the axial displacement of the inner race relative to the outer race enables the balls to roll freely in their ball tracks throughout the full axial stroke of the joint during normal operating conditions, avoiding frictional skidding of the balls which greatly decreases axial loading, friction, and noise. Under severe conditions, further stroking through axial skidding is permitted.
The invention has the further advantage of attaining such low friction axial stroking of the joint without sacrificing the true constant velocity characteristic of the joint. The constant velocity characteristic is preserved by provision of the mating spherical contact surfaces between the intermediate race and outer race, which maintain the balanced centers of curvature for all joint angles and axial stroke conditions of the joint.